1. Field of the Invention
The present invention relates to a process for the enhanced oil recovery using a displacement slug comprising an anionic surfactant and auxiliary agents in oil reservoirs having a significant ion exchange capacity.
2. Description of the Prior Art
Very many types of surfactants have been proposed for effecting enhanced oil recovery. The most widely used surfactants are of the sulfonate type, more precisely petroleum sulfonates, in the form of alkaline metal or ammonium salts.
The surfactant is usually used at a concentration higher than its critical micellar concentration. The injected micellar solutions are either aqueous solutions containing variable amounts of surfactants and, possibly, other additives such as cosurfactant, cosolvent, electrolytes, etc, or mixtures, in variable proportions, of water, electrolytes, hydrocarbons and possibly cosurfactant and/or cosolvent. In this latter case, the presence of polar-apolar molecules in a sufficient concentration, leads to the formation of transparent mixtures, generally called microemulsions.
But it is known that the efficiency of this recovery technique using such surfactants is substantially reduced when the rock contains clays having a high divalent cation exchange capacity. In fact, the reaction, for example, of calcium and magnesium ions with the sulfonates increases the surfactant consumption and makes the process inoperative, following an excessive retention due:
to the precipitation of the surfactant, PA1 to transfer of the surfactant in the hydrocarbon phase and trapping if this phase is immobilized, and PA1 to adsorption of the surfactant on the rock.
Trivalent cations exert an even greater effect but, for the sake of simplicity, only divalent cations will be mentioned in what follows.
In the conventional practice of the process, the surfactant solution to the injected is adapted to the fluids and to the temperature in the reservoir. Optimum efficiency is sought by selecting the molecular weight of the surfactant, its molecular weight distribution and its concentration in the micellar solution following tests using the oil and water from the reservoir, at the temperature of this reservoir.
However, after the composition of the micellar solution has been optimized from the point of view of lowering the interfacial tensions, under given conditions, the initial physico-chemical equilibrium between the rock and the fluids in place is modified by injecting the micellar solutions into the reservoir. These modifications tend to divert the micellar solution from its optimum efficiency and such modifications affect the salinity of the water following the increase in the divalent cation concentration due to the ion exchanges and to the increase of the solubility of the rock in water in the presence of a surfactant. Furthermore, the composition and the concentration of the surfactant are modified by dilution due to the fluids in place and to the pushing fluid and by selective adsorption of certain molecules (chromatographic effect). These changes, imposed by the porous medium, exert a dominating effect which may considerably reduce the expected efficiency of the process.
Numerous formulae have been proposed for preserving the efficiency of the micellar solution and/or reducing the retention of the surfactant.
The U.S. Pat. No. 4,232,737 describes an oil recovery method using surfactants in reservoirs containing water with a high salt content. The fluid injected contains at least two surfactants: an ionic compound, such as petroleum sulfonate, and a surfactant, such as an alcohol or alkylphenolpolyoxyethylene sulfate or sulfonate, so as to obtain solubility of the sulfonate in the water of the reservoir. In this process, several slugs of micellar solution are injected successively, followed by an aqueous solution with low salt content containing a viscosifying agent (hydrosoluble polymer). The salt and surfactant concentration decreases gradually from its maximum value in the first slug to its minimum value in the last slug.
In U.S. Pat. No. 4,074,755, during the injection of a surfactant-polymer system, the ratio of the predominant monovalent cation concentration to the square root of the predominant bivalent cation concentration is maintained at a constant value equal to that met in the reservoir. It is claimed that thus ionic exchanges are avoided in reservoirs with a high ion exchange capacity.
Despite everything, the pilot tests carried out using the processes, heretofore described and using similar methods have often led to failures. The following causes, among others, may be mentioned:
the homogeneous moving fluid optimized for given oil field conditions cannot keep its original efficiency if the conditions change as a result of the phenomena mentioned heretofore and
elimination of the formation water by previous injections, when its composition is judged unfavorable, is difficult to carry out because of the heterogeneity of the reservoirs. p Furthermore, the use of a non ionic surfactant alone, less sensitive to the salt content changes, may make the process uneconomic because of its high cost and therefore does not always form an acceptable alternative solution.
When petroleum sulfonates are used in the recovery process, they may be so called mahogany sulfonate which are essentially monosulfonates preferably soluble in the hydrocarbons. Because of their anionic character and their relatively high molecular weight, these surfactants are much more sensitive to concentration changes of the divalent cations than to those of the monovalent cations in water. Now, these changes are considerable for example when the reservoir rock has a high cation exchange capacity or when calcium carbonate for example has been dissolved following an injection of carbon dioxide.
Within certain divalent cation concentration limits, which depend on the average molecular weight of the sulfonate and on its distribution, the micellar solution tolerates these changes although the affinity of the sulfonate for the oil gradually increases. Beyond the upper limit, a qualitive change occurs which causes the formation of reverse viscous emulsions (water in oil). There follows an increase of the residual oil saturation and of the retention of the sulfonate, causing decomposition of the micellar solution.